Group III-V semiconductor materials such as gallium arsenide (GaAs), indium phosphide (InP) and their related alloys are important materials used in many different electronic and optoelectronic devices. In particular, such semiconductor materials are used in high speed transistors as well as in optoelectronic applications such as vertical-cavity surface-emitting lasers (VCSELs), in-plane lasers, and light-emitting diodes (LEDs) structured to emit light in a wide range of wavelengths from red through ultra-violet. In the following description, the term gallium arsenide material will be understood to refer to any of the above-mentioned semiconductor materials.
Semiconductor devices made from gallium arsenide materials commonly include one or more electrical contacts through which electric current received via a bonding wire is distributed across the surface of the gallium arsenide material for conduction through the bulk or thin surface layer of the gallium arsenide material. These contacts are commonly referred to as Ohmic contacts to a semiconductor device. To minimize heat generation and reduce power consumption in the semiconductor device, the electrical resistance of the electrical contact, and the voltage drop across the contact, should be minimized.
Depending on the specific device, Ohmic contacts were fabricated on either n-type (materials doped with donors) or p-type (materials doped with acceptors) gallium arsenide materials. The contact resistance on a semiconductor depends critically on the free carrier concentration (either electrons or holes) in the material. In order to obtain an Ohmic contact with low resistivity, the material has to have high carrier concentration. In gallium arsenide materials, it is well-known that highly n-type material cannot be obtained because of the limitation in n-type doping in the materials. Consequently, reliable low resistance ohmic contacts on n-type gallium arsenide are difficult to realize. The current standard state-of-the-art non-alloyed ohmic contacts to n-type GaAs utilize either Auxe2x80x94Ge alloys, multi-component metallization e.g. MoGeInW, MoGeW, NiInW or heteroepitaxial structures such as graded InxGa1-xAs. The Auxe2x80x94Ge contacts have relatively low resistivity of xcx9c10xe2x88x926 ohm-cm2. However due to the low melting point of Auxe2x80x94Ge (eutectic temperature of 360xc2x0 C.) these contacts are laterally non-uniform and have very poor surface morphology that is detrimental in optoelectronic and microscale device applications. The two main concerns with multi-component metallization are the reliability and the planar morphology of the contact. Such complications on contacts limit the miniaturization of devices. Moreover, the minimum resistance of the multi-component contacts is still limited by the near-surface doping level of the GaAs material. Contacts fabricated using graded heterojunctions can in principle achieve very low contact resistance with excellent planarity. However, such contacts can be fabricated only through an epitaxial growth process and therefore is expensive and is incompatible with current device fabrication facilities. The current invention facilitates a large enhancement of the electrical activation in a near-surface n-type GaAs layer. Such an enhancement in n-type doping will be particularly useful in the fabrication of low resistivity, nonalloyed ohmic contacts to n-type GaAs.
Ohmic contacts to n-type Group II-VI semiconductors are also possible using the same inventive steps realized for the development of the n-type GaAs.
The invention provides a method to fabricate low resistivity non-alloyed ohmic contacts on devices with a n-type gallium arsenide surface. This is achieved by the large enhancement of the n-type doping in GaAs by the co-implantation of N and a group VI donor species (S, Se and Te) in GaAs. The role of the N is to modify the conduction band structure of GaAs so that high free electron concentration can be attained by the group VI donors. This method can also be extended to any semiconductor (Group III-V) whose doping property can be modified by the introduction of an isovalent element.
The invention also provides for a method to fabricate a low resistivity non-alloyed ohmic contact with n-type Group II-VI semiconductor that is co-implanted with oxygen and a Group VII element.